The present invention is concerned generally with making nanostructured materials using plasma technologies. More particularly, the invention is concerned with a method of making a variety of stoichiometric-nanostructured materials by forming a unique xe2x80x9cactive volumexe2x80x9d in a plasma through the introduction of an oxidizing gas. The surface chemistry of the resulting nanostructured material is substantially enhanced to yield dispersion stable materials with large zeta-potentials.
Methods of plasma formation are previously known in the art and may be selected from a group of comprising radio-frequency fields, microwave discharges, free-burning electric arcs, transferred electric arcs, high-intensity lasers, capacitively coupled electro-thermal igniters, DC glow discharges, and DC cold cathode discharges.
Methods for transferring energy to a precursor material by exposing a precursor material to the energy of a plasma are previously known in the art. Precursor material may be introduced into a plasma at any point. For example, a plasma may be created by a high intensity electric arc and a precursor may be introduced at any point of the arc column. In U.S. Pat. No. 3,209,193, the precursor material is introduced into the arc column of a free-burning plasma at the anode and U.S. Pat. No. 3,900,762 describes a working embodiment of the volumetric introduction of precursor into a plasma arc.
The precursor material may also be a consumable electrode. For example, in U.S. Pat. Nos. 5,460,701 and 5,514,349, a transferred electric arc between a cathode and a consumable anode is used to generate precursors in an elongated ionized arc that extends beyond the conduction columns.
Prior art teaches that materials formed by plasma techniques may have unusual properties. But prior art does not teach the synthesis of stoichiometric-nanostructured materials with controlled surface chemistry.
Materials produced by the method of this patent have surface chemistry characterized by a high aqueous dispersion stability, a low rate of hydrolysis, and a large zeta-potential. Materials produced by the method of this patent are stoichiometricly-nanostructured by the xe2x80x9cactive volumexe2x80x9d. The xe2x80x9cactive volumexe2x80x9d is in a plasma and is created by introducing an oxidizing gas into the plasma, before the plasma is expanded into a field-free zone, either (1) in a region in close proximity to a zone of charge carrier generation, or (2) in a region of current conduction between field generating elements, including the surface of the field generating elements. Energy is transferred from the plasma to a precursor material and at least one of a stoichiometric-nanostructured material and a vapor that may be condensed to form a stoichiometric-nanostructured material are formed in the xe2x80x9cactive volumexe2x80x9d. The xe2x80x9cactive volumexe2x80x9d is the most reactive part of the plasma and material synthesized in the xe2x80x9cactive volumexe2x80x9d are stoichiometric-nanostructures with unique surface chemistry.
Stoichiometric-nanostructures or stoichiometriclly-nanostructured materials are defined as materials having controlled chemistry at the nanoscale. The chemistry of the nanostructured material may be controlled to be of full or partial stoichiometry, in the chemical sense, with respect to a reactant.
Prior art does not teach the introduction of oxidizing gas in a plasma to nanostructure materials to have unique surface chemistry. Instead prior art teaches away from the use of oxidizing gases in a plasma. For example U.S. Pat. No. 3,899,573 teaches the use of a reducing gas in the plasma created by a free-burning arc. The use of oxidizing plasma environments is conventionally discouraged because the materials used to generate the plasma are aggressively corroded. For example U.S. Pat. No. 4,642,207 discloses the use of an oxidizing plasma. But this process cannot be practiced in a manufacturing environment because aggressive corrosion rapidly renders process equipment inoperable. This is often the case even under conditions where shielding gas flows are used to protect specific process equipment as disclosed in prior art. The present invention teaches that judicious formation of an xe2x80x9cactive volumexe2x80x9d enables the use of an oxidizing environment within the conduction column of a variety of plasmas to synthesize stoichiometric-nanostructured materials with unique surface chemistry.
Prior art does not teach the importance of forming at least one of stoichiometric-nanostructured material or vapor that may be condensed to form stoichiometric-nanostructured material in the xe2x80x9cactive volumexe2x80x9d of a plasma. Instead prior art transfers energy from a plasma to precursors and forms nanoparticles by injecting at least one of a quench and a reaction gas:
after the plasma is expanded into a field-free zone; and/or
down stream from either (1) a zone of charge carrier generation, or (2) a region of current conduction between field generating elements.
U.S. Pat. Nos. 5,460,701 and 5,514,349, use a transferred electric arc between a cathode and a consumable anode to generate an elongated ionized arc that extends beyond the conduction columns and injects at least one of a quench and a reaction gas into the elongated ionized arc. Other forms of the art introduce a reactive gas down stream from the xe2x80x9cactive volumexe2x80x9d and form materials during thermal quench or gas phase nucleation. In all cases the art teaches the formation of materials in less reactive plasmas.
Experiments in our laboratory indicate the xe2x80x9cactive volumexe2x80x9d must be carefully controlled, to form before the plasma is expanded into a field-free zone, either (1) in a region in close proximity to a zone of charge carrier generation, or (2) in a region of current conduction between field generating elements, including the surface of the field generating elements, to derive the benefits of the reactive plasma and synthesize a stoichiometricly-nanostructured material with unique surface chemistry.
An object of the present invention is the development of a process for producing stoichiometric-nanostructured materials. This process comprises the steps of:
generating a plasma;
forming an xe2x80x9cactive volumexe2x80x9d through introduction of an oxidizing gas into the plasma, before the plasma is expanded into a field-free zone, either (1) in a region in close proximity to a zone of charge carrier generation, or (2) in a region of current conduction between field generating elements, including the surface of the field generating elements; and
transferring energy from the plasma to a precursor material or materials and forming in the xe2x80x9cactive volumexe2x80x9d at least one of nanoparticles and a vapor that may be condensed to form a nanoparticle.
A further object of the present invention is the production of stoichiometric-nanostructured materials with unique surface chemistry characterized by high aqueous dispersion stability, a low rate of hydrolysis, and a large zeta-potential.